Binary information is stored on magnetic media such as tapes and disks by magnetizing small areas of the magnetic surface with one of two polarities. When writing data, a current is passed through an inductive write head. A change in current from positive to negative sets the polarity of the media surface adjacent to the head to one polarity; a current transition from negative to positive sets the opposite polarity. The transition between polarities is called a flux transition. A flux transition occurring at a data bit location may represent a “one” bit, and no flux transition may represent a “zero” bit. When reading data, a magneto resistive read head passes through magnetic fields from the small magnetized areas. As the head passes through the fields, a transition from one polarity to the opposite polarity results in a resistance change in the read head.
Many modern tape drives use an array of several write heads that can operate simultaneously to write data to a tape and an array of several read heads that can operate simultaneously to read data from a tape. The magnetic heads are positioned very close to one another and the signals to one head can interfere with the signals to another head. This type of electromagnetic interference is often referred to as channel crosstalk and may arise between write heads (write-to-write channel crosstalk) or between write heads and read heads (write-to-read channel crosstalk). Channel crosstalk can arise when excessive common mode voltages are transmitted by a write current driver, which is typically a differential amplifier. The inductive write head requires and responds only to differential current provided by the write current driver. The write current driver may produce unwanted common mode voltages. The necessary component of the write signal is the differential write current and the unwanted component of the write signal is the common mode voltage. Such common mode voltages can also be the source of electromagnetic interference (EMI) or radio frequency interference (RFI) for nearby electronic devices.